(1) Field of the Invention
The present invention relates to a method of rotary piercing in seamless tube manufacturing operation by a rotary piercing process.
(2) Description of the Prior Art
The rotary piercing process (Mannesmann proces) is widely used in the manufacture of seamless steel tubes. The process comprises steps of passing billet heated to the prescribed temperatures through a rotary piercer to make them into hollow shell, rolling the hollow shell by mean of an elongator, e.g. plug mill or mandrel mill, to the desired wall thickness, and subjecting the rolled hollow shell to outside diameter sizing by means of a sizer or reducer to obtain finished tubes having the specified outside diameter (or wall thickness).
Since the present invention is directed to only the first one of these steps, namely, rotary piercing, the method of rotary piercing conventionally employed in seamless tube manufacturing is reviewed in detail below. In this connection it is to be noted that while there are known a variety of rotary piercing processes, such as Mannesmann plug mill line, Mannesmann assel mill line, Mannesmann mandrel mill line, Mannesmann pilger mill line, and Mannesmann multistand pipe mill line, the first stage of operation, namely, rotary piercing is common to all the processes.
FIGS. 7, 8 and 9 are views showing the mode of rotary piercing operation by conventional rotary piercer in plan, in side elevation, and in end elevation on the hollow-shell outlet side respectively. Rolls 71 and 71' each has a barrel shape such that its middle portion is largest in diameter and the diameter of that portion is larger than the length of the barrel, its face angle .alpha. being 2.degree..about.4.degree. on the inlet and outlet sides. The rolls 71, 71' are crosswise arranged in such manner that their axes are parallel to a vertical plane including the center of the pass line along which a billet 73 moves while being rotary-pierced until it is turned out into a hollow shell 78, or in other words, the roll axes and pass line are in parallel relation in plan as shown in FIG. 7 and that each has feed angle .beta.=6.degree..about.12.degree. (an angle which the roll axis makes with a horizontal plane including the center of the pass line) relative to the horizontal plane and directed opposite from the other. Further, as can be seen from FIG. 9 (but not from FIG. 7), between the rolls 71 and 71' there are disposed guide shoes 72, 72' in abutment relation to the outer peripheral surface of hollow shell 78 on top and bottom sides for controlling the upper and lower positions of the hollow shell 78 being progressively rotary-pierced as such.
A plug 74, supported by a mandrel 75 extending from the outlet end for hollow shell 78, has its front end positioned beyond the narrowest portion of the roll gap between rolls 71 and 71', where gorges (maximum roll-diameter portions namely minimum roll-gap portions) are positioned in opposed relation, and a little way toward the inlet end for billet 73.
When billet 73, heated to the prescribed temperature, is fed to the rotary piercer, it is driven into the roll gap between the rolls 71 and 71', which are in rotation in same direction as indicated by the arrows in FIG. 7, for rotation in the opposite direction relative to the rolls 71, 71'. The presence of an feed angle .beta. allows billet 73 to move forward; meanwhile, billet 73 is repeatedly subject to rotary forging by the rolls 71, 72'. The billet 73, under the rotary forging effect of such rolling, becomes readily centrally pierceable and is then subjected to rotary piercing and diameter expansion by the plug 74. The plug 74 is supported by the mandrel 75, and rotates freely with billet 73 and continues rotary piercing operation without retraction. Thus, billet 73 is subjected to shear deformation due to the interaction of rolls 71, 71' and plug 74 until it is turned into a hollow shell 78.
In the above described process of billet 73 being transformed into a hollow shell 78 by rotary piercing, the billet is subjected to shear deformations in three directions, viz.:
(i) longitudinal shear deformation;
(ii) surfacial shear deformation under torsion; and
(iii) circumferential shear deformation.
These modes of shear deformations are schematically illustrated in FIG. 10. Longitudinal shear deformation is a phenomenon that where the billet is assumed to consist of disc-shaped section elements having ends perpendicular to its axis as shown in FIG. 10(a), there is caused a metal flow within the billet structure which is characterized in the displacement of boundaries of individual section elements in the longitudinal direction (i.e. toward the billet inlet end of the rotary piercer) as illustrated in FIG. 10(a'). Such deformation is inevitable since the billet is subject to longitudinal elongation.
Surfacial shear deformation under torsion is a phenomenon that where the billet is assumed to have a section element parallel to the axis of the billet as shown in FIG. 10(b), there is caused a metal flow within the billet structure which deforms such section elements into one of spiral form, as shown in FIG. 10(b'). This shear deformation is undesirable because it may lead to development of outside seams (a defect resulting from a seam on the billet surface under surface torsion) on the exterior of finished tube. Circumferential shear deformation is a phenomenon that if the billet has a section element comparable to its diameter as shown in FIG. 10(c), there occurs a metal flow which causes displacement in the circumferential direction of the section element on both central and peripheral sides as illustrated in FIG. 10(c'). This shear deformation is also undesirable because it may induce formation of inside bore defects in the interior of finished tube.
The reason why surface torsional deformation or circumferential shear deformation, as the case may be, leads to the development of seams and bore on the outer or inner wall surface of the tube is that such deformation, that is, a field under shear stress, if present in the billet, will cause a crack via some inclusion in the billet and such crack will develop into seams or bore defects when the billet is rolled in the field of shear stress.
Such defects result in a decreased yield of acceptable product. Therefore, a rotary piercing method which can minimize or eliminate occurrences of surface torsional deformation and circumferential shear deformation has been much demanded in order to reduce formation of seams and bore defects in finished pipes.
The present inventor has already developed a process for manufacturing seamless steel tubes free of surface torsional deformation (as disclosed in Japanese Patent Publication No. 23473 of 1974). This process is such that where a rotary piercing mill having plate-shaped guide shoes is employed, development of surface torsional deformation and/or circumferential shear deformation as above described can be completely eliminated or substantially minimized by setting feed angle .beta. and cross angle .gamma. for rolls (cross angle is defined as an angle which the roll axis makes with a vertical plane including the center of the pass line as illustrated in FIG. 11) so as for the angles to meet the following conditions: ##EQU1## The subject matter of this earlier invention was put in practice by incorporating same into rotary piercing mills in actual operation. The result was that in rotary piercing operation with less workable materials such as Cr--Mo steel, the rate of outside seams occurrence could be remarkably lowered. At same time, it was found that the arrangement could considerably contribute toward decreasing the rate of inside bore defects occurrence.
Even with that method, however, it was found almost impossible to effectively carry out rotary piercing of extremely-hard-to-work materials having very poor hot workability, for example, stainless steels such as austenitic, ferritic, martensitic, and dual phase, and heat- and corrosion-resistant steels such as Inconel and Hastelloy, if production economy is considered. As far as the manufacture of steel tubes of these high-alloy materials is concerned, the state-of-the-art is such that there is no way but employing Ugine-Sejournet process instead of rotary piercing processes which are favorable for mass production. The reason is that the Ugine-Sejournet process hardly involves almost no possibility of surface torsional deformation and/or circumferential shear deformation being caused to the material being worked during tube manufacturing operation; therefore, there is little possibility, if any, that seams and bore defects are formed on the outside and/or inside tube surface. However, in order to carry out operation under Ugine-Sejournet process, it is necessary that a guide hole should be bored through the center of every billet along the entire length thereof by machining in advance. This naturally means increased number of operation steps and decreased efficiency and yield in billet manufacturing. It is inevitable that all these factors by the crust pressure or by the highly increased cost.
Furthermore, as recent developments in the field of steel tube manufacturing, there are two difficult problems, one arising from the material supply side and the other from the steel tube demand side. On the material supply side, the pattern of billet manufacturing is now in rapid transition from a process passing through ingot making and blooming to a process passing through continuous casting. Needless to say, billets manufactured via continuous casting are largely of such type which has center porosity; basically, these billets are not suitable for rotary piercing. On the steel-tube demand side, there is a growing tendency that high-alloy steel tubes are demanded. Referring to oil-well pipes, for example, deeper wells are rapidly increasing in number, which fact means increased load under high-concentration sulphur atmosphere, and accordingly, demand is progressively increasing for pipes of high-alloy steels, such as Incoloy and Hastelloy, which can withstand such severer conditions. As such, the emergence of a novel piercing method which permits mass production of such high-alloy steel tube under a rotary piercing process has been eagerly desired.